This study involved the development, validation, and use of the Individualized Classroom Environment Questionnaire (ICEQ), which measures classroom environment perceptions along dimensions (Personalization, Participation, Independence, Investigation, Differentiation) which differentiate individualized classes from conventional ones. The ICEQ measures student or teacher perceptions of actual or preferred classroom environment. Data from a sample of 766 students and their 34 teachers supported the ICEQ's internal consistency, discriminant validity, and ability to differentiate between the perceptions of students in different classrooms. Use of ICEQ scales as predictor variables revealed, first, that student perceptions accounted for a significant increment in the variance in affective but not cognitive outcomes and, second, that students achieved cognitive aims better when in their preferred classroom environment. Also, certain factors in the school-level environment were found to be associated with dimensions of classroom individualization. (Author/GK)